1. Field of the Invention
This invention relates to an image forming apparatus used in copying machines, facsimile machines, printers and similar devices, and more particularly to an image recording apparatus which is capable of recording multicolor images.
2. Description of Related Art
Many types of image forming apparatuses having image recording units in them are conventionally known. In these image recording units, electrode arrays called aperture electrodes are generally used.
An aperture electrode is made of a flexible insulated sheet. A plurality of minute apertures, which serve as charge passages, are formed in the flexible insulated sheet. A control electrode made of copper leaf is provided at each of the apertures.
Japanese Laid-open Patent Publication No. 2-297570 discloses an image forming apparatus which feeds a recording medium (e.g., recording paper) so that a front surface of the recording paper faces the aperture electrode, which is made of a flexible insulated sheet. A toner supply mechanism is positioned on the side of the aperture electrode opposite the recording paper. The toner supply mechanism supplies frictionally charged toner particles to the aperture electrode. A voltage applied to each of a plurality of control electrodes on the aperture electrode is individually controlled by an electrode driving circuit, based on image signals, so as to control the flow of charged toner particles through corresponding apertures. The toner particles which have passed through the apertures are attracted by an electric field produced by a back electrode which is positioned at a back surface of the recording paper. A high voltage is applied to the back electrode so that the back electrode attracts the charged toner particles. The toner particles which are attracted to the back electrode adhere onto an image forming surface, i.e., the front surface, of the recording paper and form a toner image on the recording medium.
Meanwhile, U.S. Pat. No. 5,353,050 discloses an image recording apparatus that includes three stages of image recording units, each of which corresponds to one of yellow, magenta and cyan. The three stages of image recording units are positioned along a paper-feed direction at predetermined intervals in order to form a color image. Four stages of image recording units corresponding to yellow, magenta, cyan and black may also be used. The four stages of image recording units are also arranged along the paper-feed direction at predetermined intervals.
In the conventional image forming apparatuses, the toner supply mechanisms for the three (or four) image recording units are typically driven by a single driving source, such as a motor. However, the conventional image forming apparatuses are subject to several problems.
One problem is that, in actual office environments, even if a full-color printer is installed, only one or two specific colors are predominantly used to print or record images. Because a single motor is commonly used to drive the multiple toner supply mechanisms, the toner supply mechanisms for image recording units that are not needed for a printing job are activated along with the image recording units that are actually needed for the printing job. This may cause the colors which are not required for the printing job to scatter or adhere onto the recording medium, thereby producing a stain on the output image.
Moreover, because the recording units that do not contribute to the printing operation are always driven, the durability of the recording units and the apparatus itself is deteriorated.